I like the ZS172FMM-3A (5-speed) for it's simplicity, and for a dedicated offroad bike, it's adequate. But the 6-speed version with counterbalancing is also available for about the same cost. Amazon has at least one source for the 5-speed, and ebay has a few sources including the 6-speed.
I can hear the counterbalancer linkage in the 6-speed (not loud, but audible). I am sure it is normal. That extra gear is useful, and the counterbalancing is nice for road use, but the 5speed might be a better choice for an offroad build. The choice is not that easy for me
As you can tell by the triangular right case shape, and the absence of a kick starter, this is the 6speed. It even comes with a gear shift and a starter. The real benefit of paying the additional ~$450 (more than the cost of a head) is that you can do a complete bench build and swap it in when you are ready. And the head in the complete engine includes the cam/rockers/camchain sprocket/camchain...
